When the planners of Las Vegas peered into the future in 1950, they projected that the desert city's population--then 25,000--would be lucky to break 100,000 by the end of the century. As it turned out, they were off by a factor of 19, and as you leave the sizzling Strip--the iconic center of this metropolis of 1.9 million people--for the Lake Mead reservoir, 65 miles to the northwest, you can see the source of all that growth. In a city that receives just 4 in. of rain a year, residents in the sprawling housing developments where much of the Las Vegas population lives use an average of 165 gal. of water a day--and 90% of that comes from Lake Mead, the reservoir created by Hoover Dam in 1935. Lake Mead holds Nevada's 130 billion gal. share of the Colorado River's flow, split with six other states in the West--and for decades, says Pat Mulroy, head of the Southern Nevada Water Authority, "we'd assumed it was virtually drought-proof."

It's not. Through air that shimmers in the blast furnace of a July day, you can see how far Mead's water level has fallen. White bathtub rings of mineral deposits, measuring high-water marks that grow less high every year, circle the edges of the reservoir. Today Mead's water level is 1,108 ft., down from more than 1,200 ft. in 2000. (The official drought level is 1,125 ft.) If the water continues to decline, says marine geophysicist Tim Barnett of the Scripps Institution of Oceanography, "buckle up." Barnett co-authored a study estimating a 50% chance that a combination of climate change and increased demand could render Mead effectively dry by 2021. Mulroy doubts Barnett's dire conclusion, but she knows Las Vegas--and the world beyond--faces an existential crisis over water. "This is about being able to survive as a human being," she says. (See pictures of the world water crisis.)

The reason for the world's growing water woes is evident in the numbers. The planet fairly sloshes with water--326 quintillion gal. of it--but only 0.014% of that is available for human use. The rest is nonpotable ocean water or inaccessible freshwater, most of it frozen in polar caps. And the available water we do have is far from evenly distributed. About 1.1 billion people have no access to clean water, and half the planet lacks the same quality of water that the ancient Romans enjoyed. And while the amount of water on the planet remains fixed, the number of people drawing on it does not. The world's population could grow from 6.7 billion to more than 9 billion by 2050, according to U.N. projections. Much of that growth will be in countries that are already water poor. Not only will those extra billions need to drink, they will also need to eat--and agriculture sucks up two-thirds of the world's water. They will need electricity too, and in the U.S., nearly half the water withdrawn on a daily basis is used for energy production--to turn the steam turbines in coal plants, for instance.

What's more, none of that includes a new X factor: global warming. Some areas of the world will grow wetter as a result of climate change, but others will grow dryer, and so far the drying is winning. The area of the earth's land surface classified as very dry has doubled since the 1970s; by 2050, the Intergovernmental Panel on Climate Change believes, that trend will worsen. "You do the math, and it gets a little scary," says Stuart Minchin, a water expert with the Australian Commonwealth Scientific and Research Organization. (See pictures of Australia, the driest inhabited continent.)

If the amount of water on the planet can't be changed, the way we use it has to. Water is wasted in rich countries and poor ones, in irrigation and industry, in bottles and pipes. "We're waking up," says Peter Gleick, head of the Pacific Institute, an environmental group based in Oakland, Calif. "But not fast enough."

In Australia, the wake-up call can no longer be ignored. Since 2002, the world's dryest inhabited continent has been in the grip of the worst drought in its recorded history. In Melbourne, you're no longer allowed to fill your swimming pool, and in bone-dry Brisbane, residents aren't allowed any external water use without a permit. But the real pain has been borne in the Murray-Darling River Basin in southern Australia, the heart of the country's $30 billion agricultural economy. Even in good times, Murray-Darling receives as little as 10 in. of rain a year, but 70% of the country's irrigation resources flow to the basin, creating a fertile desert able to produce 1.2 million metric tons of water-thirsty rice, among other crops

The good times, however, are gone. Last year the government allocated zero irrigation to the basin's farmers, and they produced just 18,000 metric tons of rice, the lowest yield since 1927. "No one around here has ever seen conditions like this," says rice grower Les Gordon, standing on the cracked ground of his 4,000-acre farm near the town of Barham.